1. Technical Field
This document relates to drills and/or reamers and methods for their use. For example, this document relates to novel drills and/or reamers that are well-suited for making holes in a variety of materials including, but not limited to, metals, ceramics, glass, wood, plasterboard, plastics, stone, composites, synthetics, silicon or multi-layered or hybridized substrates. In some embodiments, these drilling devices have some centers of mass that are offset from the axis of rotation. Accordingly, these drills and/or reamers may rotate and cut using a precessional pattern of motion. Precessional cutting devices will display a mechanical wave pattern in relationship to the longitudinal axis of the device. If the cutting device is fabricated from a flexible material, for example Nickel-Titanium, bodily deflection of the device may result during rotation.
2. Background Information
Industrial drills and/or reamers are cutting tools used to create cylindrical or tapered holes. Referring to FIGS. 7 and 8, a typical conventional drill or bit 204 is provided. Such a conventional drill 204 usually cuts with a circular cross-sectional profile defined by rotation of cutting edges 220 and 240. Conventional drill 204 includes a shank 203, a working body 201, and a sharp cutting tip with lips 202 that is usually conical in shape. The tip engages the work surface via sufficient axial torque to create a hole. The tip is confluent with the working body 201 of the drill 204. The working body 201 includes one or more helical flutes 230 and 250 that revolve around the central axis of the drill 204. The flutes 230 and 250 also define the leading or cutting edges 220 and 240 respectively, which are followed by margins 221 and 241 and/or the radial lands 223 and 243. In some examples, the radial lands 223 and 243 include body diameter clearance areas 225 and 245 and heels 224 and 244 respectively. The clearance areas or angles and flutes accommodate the cut chips and hauls the debris via the Archimedes principle or screw action. The flutes 230 and 250 are bridged by a metal core called the web 211.
The most common drill used in industry is known as a twist drill. Twist drill designs were described as early as Hartshorn (1882), and modified by Hanson (1904), Kallio (1960), Kim (1980) and others, and are substantially, but not exclusively, end cutting drills.
Still referring to FIGS. 7 and 8, the degree of twist of the helical flutes 230 and 250 affects the drill's 204 cutting and chip-removal properties, and is referred to as the helical angle 206, which is a measurement of the number of degrees from a perpendicular to the central axis of the drill 204. The radial lands 223 and 243, which are portions of the drill body that are not cut away by the flutes 230 and 250, create friction as the lands 223 and 243 rub against the sides of the hole. A completely cylindrical twist drill encounters greater and greater friction and axial resistance as the device advances, which can cause premature wear and often damages the drill and/or work piece. One objective of a drill design may be to improve penetration rates, which improves drill productivity. Thus, to mitigate this friction and resistance, many modern designs feature a small degree of back taper 205. Although back taper 205 is useful and a standard feature is many drill designs, back taper 205 does not completely solve the aforementioned problems of friction and axial resistance.
An additional set of problems arise from “self-excited” or “regenerative” vibrations in hole making. One form of these vibrations is known as chatter, which results from torsional-axial coupling, and is an inherent feature of many twist drills. As the drill turns or cuts, a torsional load is placed on the cutting edges causing the drill to unwind or unravel. As the drill unwinds, its axial length is increasing. This extension, however, is resisted by the downward thrust of the drill and by the drill's own torsional inertia or stiffness. Thus, a back and forth axially flexure occurs, and often manifests an irregular or wavy surface at the base of the cut (e.g., see FIG. 2). As the drill continues to rotate, it collides with these waves, and the waviness and oscillation both grow more severe. Each drill and spindle can produce a resonance or frequency that is characteristic of that system. The operator often describes this resonance as a pecking or jack-hammer effect, which creates premature tool wear, and can fracture the tool and/or damage the work piece and the machine components.
Some of the suggestions to mitigate the effects of high frequency chatter that have been offered include: 1) increased web thicknesses; 2) decreased lip relief at the tip; 3) increased feed rates; and 4) decreased helix angles. If the web is thicker, the tool will be stiffer and should chatter less. A drill with a wider lip may rub more against the bottom of the hole creating more friction, further reducing chatter. Increasing the penetration rate relative to the rpms may necessarily induce more rubbing and more friction, which may further stabilize the device. A narrower or flatter helical angle may create a greater mass axis and the drill may be stiffer and less likely to unwind. While all of these suggestions may mitigate chatter to some extent, they may simultaneously serve to reduce the cutting efficiency of the drill and/or reamer and the cutting system itself.
Another form of regenerative vibration in hole making is a lower frequency vibration that produces holes that are asymmetrical or out-of-round. While high frequency vibrations are producing chatter, this low frequency vibration is encountered as the drill sways back and forth much like a pendulum and begins to trace out a cone-shaped volume. As the cut continues, this low-frequency vibration also creates waves which amplify the severity of the vibration and a lobulated form in the hole making process emerges. A two fluted twist drill will often produce a hole that has three lobes (e.g., see FIG. 3), but larger numbers of lobes are possible. Interestingly, the positions of the lobes rotate as the depth increases. Although the center of any one lobe may correspond to the drill size, the resultant lobulated space does not, making assembly using fasteners corresponding to the size of the drill less effective.
Some of the suggestions to mitigate the effects of low frequency vibration that have been offered include: 1) increased stiffness of the drill by increasing the thickness of the web or lowering the helical angle of the flutes; 2) increased numbers of flutes; and 3) increased contact of the margin of the radial land. Adding additional cutting points may improve the orientation of the body of the drill and provide better opportunity for roundness. In some cases, increasing the width of the margin or increasing the number of margins on the tool itself or both may be helpful. There are twists drill available, with two and three lands per flute, all of which serve to dampen the effect of the vibration in the same way that reducing lip relief dampens chatter. Again, while all of these suggestions may serve to mitigate low frequency vibrations, they may simultaneously serve to reduce the cutting efficiency of the drill and/or reamer and the cutting system itself.
The web of a standard twist drill approaches 20% of the diameter of the drill itself. An increase in the web thickness of up to 40% of the drill diameter has been recommended to eliminate high frequency vibrations (chatter) and low frequency vibrations (sway). Unfortunately, an increase in web thickness from 20% to 40% will require four times more axial force to operate the drill. A precessional cutting device such as the one described herein, will require less web thickness (inherent to the design and manifest in the performance) and thereby reducing the axial force required for drill penetration improving overall efficiency and productivity of the system.
A further consideration is the configuration of the chisel tip in wide web drills. The chisel tip is the edge of the drill's end across the web that connects the cutting lips. The formation of a chip at the chisels edge is dependent on the rake angle. The smaller the rake angle the more efficient the cutting tip can be. Chip formation can still occur with a rake angle up to 40 degrees. Unfortunately, drills with wide webs (between 20-40% of the diameter of the drill) have the highest rake angle and are the least efficient. Attempts to thin the webs at the tip can only lead to premature drill wear and safety risks, and can create asymmetry in hole roundness. Another advantage of better chip formation is heat dissipation. A larger chip will absorb more heat, carrying it away from the drill and the substrate itself. Thus, within practical limits, maintaining the narrowest web possible should be the focus of any innovative drill design.
Fredrick Taylor described machine tool vibrations in 1907. However, most of the suggestions offered for mitigating machine tool vibrations have only come in the last few decades.
In attempt to resolve the problems relating to regenerative vibrations or chatter and hole asymmetry Freidli, Petigant, and Salomon (U.S. Pat. No. 4,913,603 issued Apr. 3, 1990) proposed a modification to the traditional twist design by spacing the flutes unequally around a dual web. Krieg, Gey, and George (U.S. Pat. No. 8,734,068 issued May 27, 2014) proposed a similar concept whereby the flutes were spaced unequally and whereby the web was asymmetrical.
In an effort to improve the roundness and quality of holes drilled in fiber-reinforced composites (e.g., stacked substrates), a number of inventors have described a system of cutting, collectively referred to as orbital drilling or cutting. For example, Eriksson (U.S. Pat. No. 5,641,252 issued Jun. 24, 1997) proposed a system whereby the body of the drill, which is rotating axially, is directed eccentrically circumscribing or outlining a round space. Although he does not specify the mechanism used in directing the drill eccentrically, it is accomplished with a drill that is substantially smaller than the hole itself. Tangquist, Lennart, and Backman (U.S. Pat. No. 5,685,674 issued Nov. 11, 1997) and Zackrisson, Eriksson, Jonsson, Wolf, and Roswall (U.S. Pat. No. 6,773,211 issued Aug. 10, 2004) proposed similar systems. These inventors disclosed a machine that purportedly accomplishes these objectives, and that is useful for cutting fiber-reinforced composites and metal. Again, such systems relied on a rotary drill whereby the body of the drill is displaced at a significant distance from the central axis of the hole in the work piece cutting orthogonally.
In another effort to improve roundness, Shiga, Matsushita, Fukui, Yamashita, Shimizu (U.S. Pat. No. 5,312,208 issued May 17, 1994) proposed a device called a burnishing drill or reamer, which displayed more than one diameter. A first pair of cutting edges about the first diameter extend radially outward from the foremost end of the drill, inclining axially in the rearward direction with a first cutting angle and a second pair of cutting edges about the second diameter. Its maximum external diameter is larger than the external diameter of the first cutting edges, also extending radially outward from the foremost end of the drill and inclines axially in the rearward direction with a second cutting angle. This would best be described as a combination drill and reamer.
Halley, Luner, Young, and Bayly (U.S. Pat. No. 6,379,090 issued Apr. 30, 2002) described a force balanced irregular pitch reamer that is purportedly well-suited for precision operations requiring small tolerances and wherein precision in roundness of the drilled space is required. More specifically, a reamer is described having a plurality of cutting teeth that extend outwardly at non-uniform intervals around the body portion of the drill or reamer. Each tooth generates a distinct cutting force vector. The summation of these vectors purportedly produces a balanced reamer and provides a finished hole with improved roundness. Reamers, however, are usually used in conjunction with a pilot hole and would, and therefore require a second step.
Davancens and Whinnem (U.S. Pat. No. 8,714,890 issued May 6, 2014) proposed a drill with at least two cutting flutes. The first radial distance is different from the second radial distance as measured within a first plane normal to the central axis. The length of the first cutting edge is longer than the length of the second cutting edge such that during orbital drilling the first cutting edge removes a majority of the material being machined. The first cutting edge is made of polycrystalline diamond, and the second cutting edge is made of cubic boron nitride. This method was also devised to facilitate drilling hybridized or stacked material (i.e., layers of discontinuous materials, for example, carbon fiber composite, and titanium and/or aluminum, and/or steel).
Further examples of the prior art are provided by the following patents: U.S. Pat. No. 4,149,821; U.S. Pat. No. 4,190,386; U.S. Pat. No. 4,338,050; U.S. Pat. No. 4,659,264; U.S. Pat. No. 4,740,121; U.S. Pat. No. 4,757,645; U.S. Pat. No. 4,889,456; U.S. Pat. No. 5,049,011; U.S. Pat. No. 5,312,208; and U.S. Pat. No. 5,685,674.